Processing for overlaying parts or all of a plurality of music pieces is called remixing. The types of remixing include “mashup” processing in which more sophisticated processing is performed. In the mashup processing, for example, when a music piece A and a music piece B are remixed, a new music piece is created by making the tempos of both of these music pieces (the speeds at which these music pieces are played), the numbers of beats (for example, the numbers of quarter notes) of both of these music pieces, the keys (the pitches of essential notes, such as C major or D minor) of both of these music pieces, and the chord progressions (chord changing states) of both of these music pieces conform to each other, and cutting and pasting only appropriate portions of these music pieces.
Not only a person who created a music piece by performing the mashup processing but also the person's friend sometimes enjoys listening to the music piece.
Related art documents include, for example, Japanese Unexamined Patent Application Publication No. 2006-107693.